In the MEMS nozzle arrangement described in U.S. Pat. No. 6,243,113 “Image Creation Method and Appartus” (the contents of which are incorporated herein by cross reference), an ink chamber is provided with an ink inlet and an ink ejection port, which are coaxial. The ink ejection port is provided through thermal actuator that incorporates a paddle mounted to a substrate by a passive anchor and an active anchor. The active anchor includes a resistive element that heats up upon application of a current. This heating causes expansion of the active anchor, whilst the passive anchor is sufficiently shielded from the generated heat that it remains the same length. The change in relative lengths of the anchors is amplified by the geometric position of the anchors with respect to each other, such that the paddle can selectively be displaced with respect to the ink chamber by applying a suitable drive current to the active anchor.
Upon actuation, the paddle is urged towards the ink chamber, causing an increase in pressure in the ink in the chamber. This in turn causes ink to bulge out of the ink ejection port. When the drive current is removed, the active anchor quickly cools, which in turn causes the paddle to return to its quiescent position. The inertia of the moving ink bulge causes a thinning and breaking of the ink surface adjacent the ink ejection port, such that a droplet of ink continues moving away from the port as the paddle moves back to its quiescent position. As the quiescent position is reached, surface tension of a concave meniscus across the ink ejection port causes ink to be drawn in to refill the ink chamber via the ink inlet. Once the ink chamber is full, the process can be repeated.
One difficulty with the arrangement described in this nozzle arrangement (and similar systems) is optimising resistance of the ink inlet to ink ingress. If it is too high, then the ink chamber will refill relatively slowly and the rate at which the nozzle can be fired will drop. If the resistance is too low, then the increase in ink pressure within the ink chamber will cause backflow of ink from the chamber to the inlet, thereby hampering ejection efficiency. Two ways in which resistance has been controlled to date is length and diameter of the ink supply inlet.